1. Field of the Invention
The subject invention relates to printed circuit cards for add in functions for computer based systems. More particularly, the invention relates to improved configurations for sideswipe contacts on printed circuit cards that improve safety and utility.
2. Description of the Prior Art
The PCMCIA (Personal Computer Memory Card International Association) standard was developed to provide user installed memory and I/O functions for small form factor digital computer systems. The standard specifies a card containing a printed circuit board. This product is usually referred to as a PCMCIA card or a PC card. There are three card formats: Types I, II and III. All three have external dimensions of 54 millimeters by 85.6 millimeters. Thicknesses vary. Type I is 3.3 millimeters thick. Type II is 5 millimeters thick and Type III is 10.5 millimeters thick. The standard specifies a 68 pin connector on one end. The 68 pin connector plugs into a mating connector mounted on a header which is in turn mounted to a mother board or daughter board located inside the host. The header is U shaped with the 68 pins at the base of the U. There is a wide variation of headers including headers for different thickness cards; however, the 68 pin connector is common to all PCMCIA cards.
The PCMCIA standard specifies the function of each of the 68 pins in the connector and supports either an 8 bit or 16 bit bus. There are four ground pins, two power pins and up to 3 free signal pins for additional functions.
The original PCMCIA cards were for memory addition and thus had no interaction with external devices. I/O cards were developed later to add functions such as modems, faxes, network interfaces, multi-media interfaces and sound cards. In order to handle I/O functions, a second connector was needed. However, this can only be done in a way that does not sacrifice backward compatibility. This means for example that the physical form factor cannot change and the 68 pin connector must be retained and in precisely the same location that it now commands.
By virtue of the small size of the cards, there were no standard I/O connectors or cables that were suitable. In order to solve that problem, the manufacturers of PCMCIA cards developed custom connectors and cables that mate with the card. Because they are small, it is difficult to make them robust. Because they are non-standard, they are more expensive and not readily available.
The patent application referenced in the first section, METHOD AND APPARATUS FOR PROPAGATING SIGNALS IN IC CARDS, presents a solution to the external cable problem. Rather than having a custom I/O connector on the end of the card, a "sideswipe" approach puts contacts on the side of the card. Contacts can be on one or both sides of the card. To do this, the header that the card plugs into is designed to have contacts that pick up the contacts on the side of the card when the card plugs into the host computer. The mother board in the host picks up the connections from the header and internally wires them to the back or side of the host computer where there is enough room for standard I/O connectors. Thus, the user need only plug in the card. There is no cable to forget or break.
However, there are problems with the sideswipe solution. A first problem relates to the isolation of electrical signals. That is, electrical signals on the sideswipe contact must be isolated from the chassis ground of the host computer. This is both for human safety and to protect the host hardware. For example, suppose that there is a non-sideswipe card plugged into a sideswipe header inside a host which is in turn connected to a telephone line. In order to ring a telephone, signals called tip and ring are put on the line, and these signals are about 150 volts. In addition, if lightning were to strike nearby, a very high voltage spike could appear on the phone line. Thus, provision must be made in the design of the add in PC card system so that such voltages do not appear on the system chassis.
The sideswipe concept as described in the Scheer application has contacts like a leaf spring which wipe the side of the card. If the side of the card is metal, the contacts would be in contact with the ground of the chassis. And such cards are on the market.
A second problem relates to dangers from not having the PC card fully inserted. If this happens, the first contact on the card would make contact with the next to last contact on the header, or even some other contact. In this case, there could be a host computer circuit and external-signal mismatch. For example, a tip and ring signal could end up on a logic line. If this happened, much of the circuitry in the host would likely be destroyed.